Conventionally, with the image forming apparatus of the xerography type, such as the copying machine, the laser beam printer, and the like, a recorded image is generally formed by forming a toner image on the photoreceptor drum surface according to the image information, and then transferring the toner image to the recording sheet. Specifically, with the laser beam printer, for example, the surface of the photoreceptor drum is first charged to a prescribed background potential, and by using a laser beam modulated on the image information for scanning-exposing the surface of the photoreceptor drum, a latent image is formed. And, the electrostatic latent image is developed by the developing apparatus using toner for rendering it visible as a toner image, which is then followed by transferring the toner image to a recording sheet, and by heating the recording sheet with the fuser for fixing, a recorded image is obtained.
Conventionally, for this type of image forming apparatus, which forms an image by heating and fixing, in order to prevent excessive temperature rise in the fuser resulting from the size of the recording sheet being smaller than that of the regular form, the art which, when the recording sheet as the object of image formation is smaller in size, gradually reduces the number of recording sheets per unit time according to the number of continuous recording sheets is disclosed in the patent reference 1.
According to this art, as shown in FIG. 5 as one example, when image formation is continuously performed on recording sheets of regular size the area contacting the fixing roller provided in the fuser for the recording sheet is relatively wide, and no excessive temperature rise occurs, thus image formation is continuously performed to the last page with the running time for one sheet (the period of time per recording sheet in which a prescribed location is passed at the time of image formation) and the paper standby time (the period of time per recording sheet for which the operation of carrying-out the recording sheet from the carrying-out section is halted) which have been predetermined such that a prescribed printing speed (36 ppm (pages per minute) in the same figure) is provided.
Contrarily to this, when image formation is continuously performed on recording sheets of a smaller size, the paper standby time is gradually lengthened for lowering the number of recording sheets per unit time, as the number of recorded sheets is increased in three steps of 1 to 20 sheets, 21 to 50 sheets, and more than 50 sheets, whereby the fuser is prevented from having an excessive temperature rise. In the same figure, the printing speed for the step of more than 50 sheets is fixed because, during the paper standby time, e, which is set for this step, the temperature of the fixing roller is sufficiently lowered, and thereafter on whatever number of sheets image formation is continuously performed, no excessive temperature rise will occur.    Patent reference 1: Japanese Patent Laid-Open Publication No. 9-218608